diff --git a/apps/api/src/services/CampaignService.ts b/apps/api/src/services/CampaignService.ts
index 7e8dfde..2b9b952 100644
--- a/apps/api/src/services/CampaignService.ts
+++ b/apps/api/src/services/CampaignService.ts
@@ -1,17 +1,18 @@
 import type {Campaign, Contact, Prisma} from '@plunk/db';
-import {CampaignAudienceType, CampaignStatus} from '@plunk/db';
+import {CampaignAudienceType, CampaignStatus, EmailSourceType} from '@plunk/db';
 import type {FilterCondition} from '@plunk/types';
 
 import {prisma} from '../database/prisma.js';
 import {HttpException} from '../exceptions/index.js';
 import {buildEmailFieldsUpdate} from '../utils/modelUpdate.js';
 
+import {BillingLimitService} from './BillingLimitService.js';
 import {DomainService} from './DomainService.js';
 import {EmailService} from './EmailService.js';
 import {NtfyService} from './NtfyService.js';
 import {QueueService} from './QueueService.js';
 import {SegmentService} from './SegmentService.js';
-import {DASHBOARD_URI} from '../app/constants.js';
+import {DASHBOARD_URI, STRIPE_ENABLED} from '../app/constants.js';
 import {sendRawEmail} from './SESService.js';
 
 const BATCH_SIZE = 500; // Number of emails to process per batch (increased for better performance)
@@ -324,6 +325,24 @@ export class CampaignService {
       throw new HttpException(400, 'Campaign has no recipients');
     }
 
+    // Check billing limits before scheduling/sending the campaign
+    // This ensures users cannot schedule campaigns that would exceed their quota
+    if (STRIPE_ENABLED) {
+      const limitCheck = await BillingLimitService.checkLimit(projectId, EmailSourceType.CAMPAIGN);
+
+      // If there's a limit set, verify the campaign won't exceed it
+      if (limitCheck.limit !== null) {
+        const projectedUsage = limitCheck.usage + recipientCount;
+
+        if (projectedUsage > limitCheck.limit) {
+          throw new HttpException(
+            403,
+            `Cannot ${scheduledFor ? 'schedule' : 'send'} campaign: would exceed billing limit. Current usage: ${limitCheck.usage}/${limitCheck.limit} emails, campaign recipients: ${recipientCount}. Upgrade your plan or reduce campaign recipients.`,
+          );
+        }
+      }
+    }
+
     if (scheduledFor) {
       // Schedule for later
       if (scheduledFor.getTime() <= Date.now()) {
diff --git a/apps/api/src/services/__tests__/CampaignService.test.ts b/apps/api/src/services/__tests__/CampaignService.test.ts
index 563a5a3..23eda10 100644
--- a/apps/api/src/services/__tests__/CampaignService.test.ts
+++ b/apps/api/src/services/__tests__/CampaignService.test.ts
@@ -356,4 +356,227 @@ describe('CampaignService', () => {
       expect(matching).toBe(5);
     });
   });
+
+  describe('send', () => {
+    it('should throw error when campaign has no recipients', async () => {
+      // Create campaign with no contacts in project
+      const campaign = await factories.createCampaign({
+        projectId,
+        status: CampaignStatus.DRAFT,
+      });
+
+      await expect(CampaignService.send(projectId, campaign.id)).rejects.toThrow('Campaign has no recipients');
+    });
+
+    it('should send campaign successfully when recipients are under billing limit', async () => {
+      // Set billing limit for campaigns
+      await prisma.project.update({
+        where: {id: projectId},
+        data: {billingLimitCampaigns: 100},
+      });
+
+      // Create 5 subscribed contacts
+      for (let i = 0; i < 5; i++) {
+        await factories.createContact({
+          projectId,
+          subscribed: true,
+        });
+      }
+
+      // Create campaign targeting all contacts
+      const campaign = await factories.createCampaign({
+        projectId,
+        status: CampaignStatus.DRAFT,
+        audienceType: CampaignAudienceType.ALL,
+      });
+
+      // Should send successfully (5 recipients < 100 limit)
+      const sentCampaign = await CampaignService.send(projectId, campaign.id);
+
+      expect(sentCampaign.status).toBe(CampaignStatus.SENDING);
+      expect(sentCampaign.totalRecipients).toBe(5);
+    });
+
+    it('should throw 403 error when campaign would exceed billing limit', async () => {
+      // Set billing limit for campaigns to 10
+      await prisma.project.update({
+        where: {id: projectId},
+        data: {billingLimitCampaigns: 10},
+      });
+
+      // Create 5 existing campaign emails (usage = 5)
+      const contact = await factories.createContact({projectId, subscribed: true});
+      for (let i = 0; i < 5; i++) {
+        await factories.createEmail({
+          projectId,
+          contactId: contact.id,
+          sourceType: 'CAMPAIGN',
+        });
+      }
+
+      // Create campaign with 10 subscribed contacts (would result in 15 total)
+      for (let i = 0; i < 10; i++) {
+        await factories.createContact({
+          projectId,
+          subscribed: true,
+        });
+      }
+
+      const campaign = await factories.createCampaign({
+        projectId,
+        status: CampaignStatus.DRAFT,
+        audienceType: CampaignAudienceType.ALL,
+      });
+
+      // Should throw error because 5 + 11 = 16 > 10 limit (11 contacts: 1 from email creation + 10 new)
+      await expect(CampaignService.send(projectId, campaign.id)).rejects.toThrow(/exceed billing limit/i);
+    });
+
+    it('should throw 403 error when scheduled campaign would exceed billing limit', async () => {
+      // Set billing limit for campaigns to 20
+      await prisma.project.update({
+        where: {id: projectId},
+        data: {billingLimitCampaigns: 20},
+      });
+
+      // Create 15 existing campaign emails
+      const contact = await factories.createContact({projectId, subscribed: true});
+      for (let i = 0; i < 15; i++) {
+        await factories.createEmail({
+          projectId,
+          contactId: contact.id,
+          sourceType: 'CAMPAIGN',
+        });
+      }
+
+      // Create campaign with 10 subscribed contacts (would result in 25 total)
+      for (let i = 0; i < 10; i++) {
+        await factories.createContact({
+          projectId,
+          subscribed: true,
+        });
+      }
+
+      const campaign = await factories.createCampaign({
+        projectId,
+        status: CampaignStatus.DRAFT,
+        audienceType: CampaignAudienceType.ALL,
+      });
+
+      const scheduledFor = new Date(Date.now() + 60 * 60 * 1000); // 1 hour from now
+
+      // Should throw error when scheduling because 15 + 10 = 25 > 20 limit
+      await expect(CampaignService.send(projectId, campaign.id, scheduledFor)).rejects.toThrow(
+        /Cannot schedule campaign.*exceed billing limit/i,
+      );
+    });
+
+    it('should send campaign successfully when billing limit is null (unlimited)', async () => {
+      // Set billing limit to null (unlimited)
+      await prisma.project.update({
+        where: {id: projectId},
+        data: {billingLimitCampaigns: null},
+      });
+
+      // Create 1000 subscribed contacts
+      for (let i = 0; i < 1000; i++) {
+        await factories.createContact({
+          projectId,
+          subscribed: true,
+        });
+      }
+
+      const campaign = await factories.createCampaign({
+        projectId,
+        status: CampaignStatus.DRAFT,
+        audienceType: CampaignAudienceType.ALL,
+      });
+
+      // Should send successfully (no limit)
+      const sentCampaign = await CampaignService.send(projectId, campaign.id);
+
+      expect(sentCampaign.status).toBe(CampaignStatus.SENDING);
+      expect(sentCampaign.totalRecipients).toBe(1000);
+    });
+
+    it('should allow campaign that exactly reaches billing limit', async () => {
+      // Set billing limit for campaigns to 10
+      await prisma.project.update({
+        where: {id: projectId},
+        data: {billingLimitCampaigns: 10},
+      });
+
+      // Create 5 existing campaign emails
+      const contact = await factories.createContact({projectId, subscribed: true});
+      for (let i = 0; i < 5; i++) {
+        await factories.createEmail({
+          projectId,
+          contactId: contact.id,
+          sourceType: 'CAMPAIGN',
+        });
+      }
+
+      // Create 4 more subscribed contacts (total of 5 with the one above: 1 + 4 = 5)
+      for (let i = 0; i < 4; i++) {
+        await factories.createContact({
+          projectId,
+          subscribed: true,
+        });
+      }
+
+      const campaign = await factories.createCampaign({
+        projectId,
+        status: CampaignStatus.DRAFT,
+        audienceType: CampaignAudienceType.ALL,
+      });
+
+      // Should send successfully because 5 + 5 = 10 (exactly at limit)
+      const sentCampaign = await CampaignService.send(projectId, campaign.id);
+
+      expect(sentCampaign.status).toBe(CampaignStatus.SENDING);
+      expect(sentCampaign.totalRecipients).toBe(5);
+    });
+
+    it('should throw error when campaign has already been sent', async () => {
+      const campaign = await factories.createCampaign({
+        projectId,
+        status: CampaignStatus.SENT,
+      });
+
+      await expect(CampaignService.send(projectId, campaign.id)).rejects.toThrow(
+        'Campaign has already been sent or is currently sending',
+      );
+    });
+
+    it('should schedule campaign successfully when recipients are under billing limit', async () => {
+      // Set billing limit for campaigns
+      await prisma.project.update({
+        where: {id: projectId},
+        data: {billingLimitCampaigns: 50},
+      });
+
+      // Create 10 subscribed contacts
+      for (let i = 0; i < 10; i++) {
+        await factories.createContact({
+          projectId,
+          subscribed: true,
+        });
+      }
+
+      const campaign = await factories.createCampaign({
+        projectId,
+        status: CampaignStatus.DRAFT,
+        audienceType: CampaignAudienceType.ALL,
+      });
+
+      const scheduledFor = new Date(Date.now() + 60 * 60 * 1000); // 1 hour from now
+
+      // Should schedule successfully (10 recipients < 50 limit)
+      const scheduledCampaign = await CampaignService.send(projectId, campaign.id, scheduledFor);
+
+      expect(scheduledCampaign.status).toBe(CampaignStatus.SCHEDULED);
+      expect(scheduledCampaign.scheduledFor).toEqual(scheduledFor);
+      expect(scheduledCampaign.totalRecipients).toBe(10);
+    });
+  });
 });